A light emitting diode (LED) light emitting device is mainly constructed from a combination of an LED chip as an excitation light source and phosphors, and is capable of realizing a variety of luminescent colors by means of the combination.
It is general, in white LED light emitting devices that emit white light, to use a combination of an LED chip that emits light having a wavelength in the region from ultraviolet to blue, and a phosphor. An example of such a combination may be a combination of an LED chip that emits blue light, and a phosphor mixture that emits light by absorbing the light emitted from the LED. In this case, a phosphor that emits yellow light, which is a complementary color of blue, is mainly used as the phosphor, and the combination is used as a light emitting device which emits quasi-white light on the whole. In addition to that, there has also been developed a three-wavelength type white LED light emitting device which uses a combination of an LED chip that emits blue light, a green or yellow phosphor and a red phosphor.
As one of red phosphors, 3.5MgO.0.5MgF2.GeO2:Mn is known. Furthermore, there has also been suggested a white LED light emitting device which combines the red phosphor mentioned above, an yttrium-aluminum-garnet (YAG)-based yellow phosphor, and a blue semiconductor light emitting element. Phosphors used in white LED light emitting devices are required to exhibit large absorption of the light emitted from an LED chip, which serves as an excitation light source, and efficient emission of visible light. When it is intended to use an LED light emitting device for the purpose of illumination applications or display backlight applications, a light emitting device having two or more kinds of phosphors is preferred, from the viewpoints of characteristics such as color rending properties, which represent the visuality of an object, and the color gamut which expresses rich colors.
In addition, there has been a demand in recent years for further enhancement of the luminance of LED light emitting devices, and attention has been paid to high power type LED light emitting devices. High power type LED light emitting devices generally require larger input electric power as compared with conventional LED light emitting devices. As a result, high power type LED light emitting devices have a strong tendency that the temperature in the vicinity of the LED chip increases even during a short time driving. When such a temperature increase occurs, the phosphor in the phosphor layer generally experiences a change in the luminescence properties. Particularly, in the case of a white LED light emitting device, as a result of a decrease in the luminescence intensity of the phosphor, there occur problems such as a decrease in the luminance of the light emitted from the light emitting device, and a color shift in the light emitted from the light emitting device, which is caused by a disruption in the balance between the LED chip and the luminescence intensity of the phosphor.